The present invention relates to a construction of a rubber tube incorporated into a pipe-line for dredging work, and more particularly, to a rubber tube suitable for use in dredging crushed pieces of coral in a coral reef and crushed stones with sharp edges.
In a pipe-line for dredging work including a suction line a delivery line used for dredging work in a river or sea, a rubber tube having a relatively large diameter is interposed between iron pipes to impart flexibility to said lines. However, where work is performed in the coral reef or in a sea or river containing crushed stones with sharp edges, crushed pieces of coral and solid particles such as crushed stones in a current of sand and earth are transported under high pressure within the pipe-line and violently impinge upon the inner peripheral surface of the rubber tube. Therefore, the inner peripheral surface of the rubber tube become cut and torn, and torn off by said particles, as a result of which the rubber tube becomes worn soon. Under these circumstances, various improvements have been heretofore proposed to prevent damages caused by such crushed pieces.
For example, in Japanese Utility Model Application Laid-Open No. 55-34075 which was filed by the same assignee as the present application, there is proposed a rubber tube for dredging work in which a plurality of metal rings having a tapered inner surface are embedded in an inner peripheral surface of the rubber tube in a suitably spaced relation. This rubber tube is provided with a connecting fitting 1 at an axial end thereof as shown in FIG. 1, and metal rings 2 are embedded in the inner peripheral surface of the connecting fitting in an axially spaced relation. The metal rings 2 of the rubber tube 3 each has an inner peripheral surface 4 tapered towards one direction in an axial direction, said inner peripheral surface 4 being protruded from the inner peripheral surface of the rubber tube 3. Solid particles such as crushed pieces of coral contained in the current of earth and sand impinge upon the tapered inner peripheral surface 4 of the metal ring 2 and are knocked off in an axial direction, after which they are carried away by high pressure fluids and discharged into the succeeding iron pipe. This rubber tube 3 for dredging work can minimize the chance for the solid particles 5 to impinge directly on the inner peripheral surface of the rubber tube by the provision of the metal rings 2, thus considerably improving the cut and tear and wear resistance of the inner peripheral surface of the rubber tube.
However, since extremely high internal pressure is applied to the rubber tube 3 during the use, the rubber itself is radially bulged and end surfaces 6 of both axial ends of the metal rings 2 embedded into the rubber tube 3 are often peeled off from the surface in contact with the rubber. It is assumed that the occurrence of such peel-off and damage of the metal rings 2 mainly results from a short of adhesive force which occurs during the step of manufacturing the rubber tube 2. That is, the method of manufacturing the rubber tube will be briefly explained. First, a plurality of metal rings 2 are positioned on a mandrel in a required spaced relation. Next, after unvulcanized rubber sheets 7 and strengthening cloth layers 8 have been successively laminated, an outer peripheral surface thereof is firmly rolled by a tape-like cloth 9 having a relatively large width. A reference numeral 10 designates a metal wire for initially tightening the structure, which is then charged into a vulcanizing can for vulcanization under predetermined vapor pressure. As may be understood from such a manufacturing operation, in the tightening of the tape-like cloth 9, a tightening force with respect to the outer peripheral surface of the metal rings 2 is different from a tightening force with respect to the rubber sheet layers 7 to produce an unbalance of the unbalanced tightening forces. Thus, tightening force are applied to the structure at a boundary line, that is, at both end surfaces 6 of the metal rings 2, and it is considered that the peel-off and damage as described above occur by a boundary tension resulting from unbalanced pressures applied from inside and outside of the rubber tube 3 or bending deformation of the rubber tube 3 when used.
The same assignee as the present application has proposed in Japanese Utility Model Application Laid-Open No. 55-85163 a rubber tube for dredging work in which each metal ring is provided with a tapered cylindrical member, which has a longer axial length than that of the metal ring and which is fixedly mounted by weling or the like on an inner peripheral surface of the metal ring. This rubber tube will be described with reference to FIG. 2 in accordance with the manufacturing operation. Here, the same elements as those of the aforementioned prior art embodiment of FIG. 1 bear the same reference numerals. A rubber tube 11 is made by mounting unvulcanized rubber sheets 7, strengthening cloth layers 8 and metal rings 12 whose inner peripheral surface is flat on a mandrel and thereafter vulcanizing and molding the same. The thus vulcanized and molded rubber tube 11 is removed from the mandrel, and then a tapered cylindrical member 13 having a longer axial length than the metal ring 12 is mounted on the metal ring 12. This mounting is achieved by securing an outer peripheral portion having a large diameter at one end of the cylindrical member 13 to an inner peripheral surface of the metal ring 12 by means of spot welding or the like. Accordingly, it requires trouble in molding the rubber tube 11 for dredging work, and in addition, since the metal rings 12 are embedded into the rubber sheet 7, the high temperature heat generated during the welding is directly transmitted to the rubber elastic material in the neighbourhood, of the welds the rubber material in the neighbourhood is heat-deteriorated to possibly lower the adhesive strength of the metal rings 12. Furthermore, the possible occurrence of the peel-off and damage as described above still remains.